The incident took place on the intervening night of Thursday and Friday.
The ongoing TROPEX exercise was recently witnessed by Defence Minister Manohar Parrikar.

"In a minor incident off the west coast of India, a fishing vessel has hit the periscope of submarine which was involved in an exercise. The exercise was being conducted in pitch dark conditions as required by the exercise prerequisites," Navy spokesperson D K Sharma said.

He added that there was no major damages to the submarine and it has since returned to the Mumbai dock and repairs would be effected soon. He stressed that all relevant authority including the Defence Ministry was immediately informed.

This is the third time the submarine has been involved in an incident since 2008.

In 2008, it had collided with a foreign vessel while taking part in a fleet level war game and last year it had run aground due to low tide.

Meanwhile, asked how could a fishing vessel hit the periscope when an exercise was on, defence sources said the exercise was about getting divers out without being noticed.     

"It required the submarine to be under water near the coast with no lights or sound. Since there are many fishing vessels around, one of them hit the periscope," they said.

The submarine was immediately brought back to the dock and the "misalignment" of the periscope is being rectified after which it was back in the waters, the sources said.

The government had in December informed the Parliament that there have been 24 mishaps involving Indian naval vessles since 2011, claiming the lives of 22 personnel besides four others going missing.

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